1. Field of the Invention
The present invention relates generally to testing printed circuit board assemblies, and more particularly to fixtures used in "bed-of-nails" type printed circuit board testing apparatus.
2. Description of the Related Art
Printed circuit boards are an increasingly common component in all types of electronic devices. These printed circuit boards are relatively easy and inexpensive to manufacture, install, and use. Printed circuit boards eliminate the need for a chassis full of loose and tangled wire connections. As a result, complex electronic equipment can be made less expensively and much more compactly.
Printed circuit boards generally include a board constructed from an insulating material such as a glass fiber reinforced epoxy resin. The board can be rigid or it can be relatively flexible. A plurality of electrically conductive circuitry tracks are formed onto the board by printing techniques. These tracks are then connected to various electrical components when the components are affixed to the printed circuit board to provide a printed circuit board assembly.
Printed circuit board assemblies require testing after the components have been affixed on the printed circuit board to determine interconnectional continuity, board functionality, and proper placement and connection of components. Several different approaches have been developed for testing printed circuit board assemblies including, for example, functional testing, in-circuit testing, and continuity testing.
A common in-circuit testing technique to individually test each of the components on a printed circuit board is a "bed-of-nails" technique. The bed-of-nails technique uses fixed position spring-loaded probes to establish an electrical contact between test points on a printed circuit board and the test equipment. The contact may alternately be against the leads of each component to access and test the individual component. The testing equipment measures an integrity parameter at each predetermined location and compares it to a standard value. In this manner, non-functioning components can be readily identified and replaced, thereby preventing an entire board from being wasted. This process works especially well for common digital integrated circuits, the tests for which can be programmed once, stored in a library, and used when needed.
Contacting the various points of the printed circuit board is typically accomplished by placing the printed circuit board on a test fixture device and bringing the printed circuit board into contact with the test probes. An electrical charge is applied at points along the circuitry, and the resulting electrical characteristics at each of the predetermined locations are measured using the probes and compared by the test equipment with the standard values.
Bed-of-nails test fixtures generally include a number of fixed-position springloaded pins (the "nails") supported within a frame (the "bed") and connected to the test equipment via individual cables. Each particular bed-of-nails fixture is specially designed and manufactured with an appropriate number of pins suitably positioned to perform specific tests on a particular type of printed circuit board. The design and manufacture of such fixtures is well established in the art. In general, when the board is first placed in the test fixture, it is not in contact with the test probes. The board is brought into contact with the test probes.
A known approach for bringing the printed circuit board into contact with the test probes is a vacuum assist method. The vacuum assist method forces connections between the printed circuit board circuitry and the circuitry of the tester bed-of-nails by creating a vacuum between the printed circuit board and the test probes. The vacuum provides the large amount of force necessary to overcome the spring pressures of a hundred or more individual probes. Most commercially available test fixtures use a partial vacuum induced in the space between the board and the test bed to bring the board into contact with the probes. To achieve this, a flexible airtight seal is provided around the board. The seal is generally provided by a rubber seal with a spring means of returning the board to the upper position when the vacuum is released.
Although the vacuum assist method is effective in holding the printed circuit boards in place, exclusive use of the vacuum assist method may cause undesired board flex. Excessive board flex is especially likely if the test probes are not evenly distributed about the total printed circuit board area. Test probe density is often higher in one area of the printed circuit board than in other areas of the printed circuit board. Board flex and subsequent associated failures can increase to a point of creating long-term reliability problems as a result of using vacuum pull down methods.
There is a trend to use mechanical hold-down fixtures to increase throughput and reliability of the bed-of-nails fixture. A mechanical hold-down fixture uses a hold-down gate, which includes a framework which is hinged and attached to a bed-of-nails fixture. The mechanical hold-down fixture pivots for inserting the printed circuit board into and removing the printed circuit board from the test fixture. The mechanical hold-down gate includes a plurality of parallel, sturdy bars. Hold-down fingers are located in channels within the bars.
The mechanical hold-down devices have built-in spacing restrictions. The width and spacing of the parallel bars define the spacing of the fingers. The width and spacing of the bars often prevent the fingers from being placed where they are desired or required for printed circuit board testing. Therefore, the mechanical hold-down devices and the fingertips thereof may not be in the desired positions to properly hold the printed circuit board in place.